Hydraulic excavators and other construction machines have a diesel engine as a driving source for the machine. Regulations on the amount of particulate matter (PM) emitted from the diesel engine are being strengthened each year along with those of NOx, CO, HC, and the like. Exhaust gas cleaning systems for trapping PM with a filter called the diesel particulate filter (DPF), and reducing the amount of PM emitted to the outside, are known to be effective under the increasingly stringent regulation situation (refer to Patent Documents 1 to 3). In these exhaust gas cleaning systems, since increases in the amount of filter-trapped PM clog the filter, thus increase the engine exhaust gas pressure, and result in undesirable events such as decreases in fuel efficiency, the filter-trapped PM is burned in appropriate timing to remove the PM clogging the filter, and hence to regenerate the filter.
The filter is usually regenerated using an oxidizing catalyst. The oxidizing catalyst may be disposed at the upstream side of the filter or supported by the filter directly or provided in both forms. In all of the three cases, to activate the oxidizing catalyst, it is necessary that the engine exhaust gas temperature be higher than the activation temperature of the oxidizing catalyst. Therefore, the technology called “forced regeneration” that forcibly increases the exhaust gas temperature above the activation temperature of the oxidizing catalyst is applied. Forced regeneration uses, for example, one of two methods: increasing the exhaust gas temperature by, as disclosed in Patent Document 1 or 2, conducting subsidiary injection (post-injection) to inject the fuel during the expansion stroke of the engine after in-cylinder main injection, or as disclosed in Patent Document 3, increasing the exhaust gas temperature by injecting the fuel into the exhaust gases flowing through a regenerating fuel injector provided in the exhaust pipe.
Also, forced regeneration of the filter has two forms: manual regeneration, which is started upon operator input of an operating instruction; and automatic regeneration, which is started automatically. Traditionally, as described in Patent Documents 1 and 2, regeneration in these forced regeneration forms is started in the timing that the estimated amount of PM deposition in the filter (i.e., the estimated accumulation level) reaches the previously set maximum permissible accumulation level of PM. In this case, as in the technique of Patent Document 1, the amount of PM deposition computed is generally based upon the sensed value of a differential pressure existing across the filter. Another computing method is by, as in the technique of Patent Document 2, reading in the output values of a speed sensor, a load sensor, and a temperature sensor, then after calculating the amount of PM emission, We, and the amount of PM combustion, Wc, deriving the amount of PM deposition, Wa, from Wa=We−Wc, and adding Wa1, the previous value of Wa, to newly obtain an integrated deposition level Wa1.    Patent Document 1: JP-A-2005-282545    Patent Document 2: JP-A-2001-280118    Patent Document 3: JP-A-2007-170382